Referring to FIG. 10, a conventional monolithic iron stator core 2 for a brushless permanent magnet motor includes an annular hub 3 having a central circular hole 4 and a plurality of stator teeth 5 extending radially outward from the hub. Each adjacent pair of stator teeth 5 defines a slot 6 for receiving wire (not shown) wound around the corresponding teeth. Each tooth 5 includes a free end or tip 7 having an arcuate outer surface 8. In the illustrated core 2, the arcuate surface 8 of each stator tooth 5 is concentric with the circular hole 4. There is a narrow gap 9 between each adjacent pair of free end 7 of the stator teeth 5. During assembly of the stator, a winding needle (not shown) passes through the gaps 9 as it winds electrically conductive wires around each tooth 5 to form stator windings.
Due to the width of the winding needle, the narrow gap 9 must not be made too narrow. Yet, the gap 9 should not be too wide. If the gap 9 is too wide this will have an undesirable impact on performance of the motor. Therefore, the conventional monolithic stator core 2 has the following disadvantages: It is difficult to wind stator windings around the teeth 5; there is a low slot fill factor; and there is low utilization of material.
Although a high slot fill factor can be achieved by conventional segmented stator structures, the assembling process after wire winding is complicated because the multiple stator segments have to be assembled together to form the stator core and numerous wire connections may be needed to connect the stator windings.
A variety of structures have been proposed to solve the problems associated with a conventional monolithic stator cores. For example, in the stator structures disclosed in U.S. Pat. No. 6,583,530 B2, U.S. Pat. No. 6,404,095 B1, and U.S. Pat. No. 6,400,059 B1, each tooth is wound individually while it is separate from the rest of the stator, and then the teeth are connected to segmented stator yokes after the windings are already on the teeth and assembled to form a stator using interlocking structures on the teeth. The assembly process for these stators is complicated, adding to manufacturing costs.
U.S. Pat. No. 6,781,278 B2 discloses another example of a stator structure. Wire is wound around electrically insulating tooth rings to form the stator windings. Then a stator tooth is inserted into each tooth ring and secured to the stator body by means of a positioning pin to form the stator. This stator structure is difficult to assemble because all the teeth have to be inserted into corresponding slot structures in the stator body to assemble the stator and the positioning pins have to be inserted into a notch formed between the stator body and each tooth.
In the stator structures disclosed in US 2009/0183357 A1 and U.S. Pat. No. 7,538,467 B2, two initially separate stator magnetic yoke modules made from a moldable magnetic powder material are joined to form a stator core. After the yoke modules are joined, stator windings are wound around the teeth of the stator core. The yoke modules are configured and combined in such a way to produce a stator core that is skewed to reduce cogging. This stator core is costly to assemble and does not solve any of the problems arising from the difficulty in passing a winding needle through the relatively narrow gaps between the free ends of adjacent stator teeth to wind the stator windings around the teeth.
In the stator structures disclosed in U.S. Pat. No. 7,679,255, WO 2004/098023 A1, and WO 2006/018346 A1, the stator is formed by disposing multiple modules adjacent one another in an axial direction and combining them to form a stator core. Each module is formed from a moldable soft magnetic powder iron composite and includes a ring-shaped magnetic yoke and poles extending radially from the yoke. Each module includes a fraction of the total number of poles and the stator windings can be wound around the poles before the modules are combined.
The present invention is directed to overcoming one or more of the problems set forth above.